Examples of ladders requiring securement at a distal or upper end thereof are shown, for example, in U.S. Pat. No. 1,205,594 issued to Close and titled “Scaling Ladder,” U.S. Pat. No. 2,448,716 issued to Hurd and titled “Hook Attachment for Ladders,” U.S. Pat. No. 4,279,327 issued to Warren and titled “Ventilating Ladder,” U.S. Pat. No. 4,531,613 issued to Keigher for a “Firefighter's Ladder,” and U.S. Pat. No. 6,167,988 issued to Frodge et al. for a “Lineman's Ladder Stabilizer”. In each of these patented inventions, hooks are provided at an upper end of each of the left and right ladder rails to secure an upper end of the ladder rails to an object being worked on, such as a roof top, support arm, or cable. In various aspects, these hooks are provided with sharp points to permit the tip of the hook to penetrate a surface and provide further stability, a feature that is particularly beneficial for firefighter's.
Additional examples of ladders requiring securement at a distal or upper end thereof are found in the posting ladder art, such as the Stokes Ladders, Inc., Model Nos. 1300-1400 Billboard Posting Ladders or Featherlite Industries, Ltd. 6700 series billboard ladders. Similar posting ladders are manufactured by Formetco and Green Bull. Conventional posting ladders 10 typically come in either a straight or tapered form, such as shown in FIGS. 1(a)–(b), respectively. These ladders 10 are typically hooked over the top of a billboard 20 and the bottom end of the ladder is permitted to hang freely without vertical support, such as shown in FIG. 1(c). Often the bottom end of the ladder is displaced horizontally from the billboard by a catwalk 30, which projects laterally from the bottom of the billboard 20 generally between about one and six feet, six feet being typical. The hooks 40 used in these conventional posting ladders are generally made from aluminum bar or tubing and are generally about ¾″ diameter (solid hook) and a 1⅛″ diameter (tubular hook).
FIGS. 2(a)–2(b) respectively show an isometric view of a conventional ladder hook 40 configuration and a corresponding view showing the stress field in a conventional, solid ladder hook during application of a point force in the middle of the hook. For an applied load of 2,500 lbf. applied to a midpoint of the hook 40, as shown by the upwardly-pointing arrow, the stress between the tip of the hook 42 and the point of force application (i.e, an arc subtending an angle of roughly 90°, wherein the 0° point is defined at the tip of the hook) is substantially zero, as is expected. Along the remainder or intermediate portion 43 of hook 40 between the point of force application and the base portion of the hook 44, disposed roughly 180° to the tip of the hook, the stress rises to a value of about 2.5×105 psi (250,000 psi) over a broad region and achieves a maximum tensile stress of between about 2.5×105 psi to 3.15×105 psi (280,000–315,000 psi) occurring along an inner surface of the hook. Stresses around the uppermost attachment hole 45 (e.g., screw or rivet hole) which receives the mechanical connection member (e.g., screw or rivet) are uniformly in the range of about 1.4×105 psi to 1.7×105 psi (140,000–170,000 psi).
However, failure of a single hook instantly places significant stresses on the remaining hook, resulting in strains that predictably lead to the rapid failure of the remaining hook. In many instances, such failure leads to disengagement of the ladder from the adjacent surface and collapse of the ladder, as the hooks are typically the only means by which the ladder is vertically secured. In the case of posting ladders, for example, the ladder would simply slide off of the billboard and catwalk to the ground below. Clearly, should a ladder hook fail, the risk for injury to lineman, firefighters and/or fire victims, and billboard workers is significant.
Thus, a need exists for improvement to the hooks used to secure the general class of ladders requiring securement at a distal or upper end thereof to further enhance the structure and performance of the hook and to correspondingly provide enhanced stability of the folding ladder.